Maybe parts of it, the intro music would just be swapping the mp3 in 'music' folder, loading screen for tracks just swap them for the existing ones in GSC track folders, the U.I in Marcas has the multiplayer function off, this might be complicated for most people? Study the U.I and related folders, do a test install and try - then come back and tell us all how to did it . I think also the relevent settings positioning in the U.I (for things like car setting options position) would all need editing, for this you need the right software and some working knowledge. IMO it is better left as it is.
The custom U.I already is too far removed from GSCE aesthetics, too rFactor-ish. I will say though I love the Marcas U.I colours and feel.